Strategies of survival and resource exploitation in the Antarctic fellfield ecosystem

Abstract

Antarctic fellfields present organisms with a heterogeneous habitat characterised by a wide variety of environmental stresses. These include low temperatures, limited moisture availability, frequent and often rapid freeze-thaw and hydration-dehydration cycles, exposure to high photosynthetic photon flux density and ultraviolet (uv) irradiance, seasonal snow cover, high winds, cryoturbation and, depending on location south of the Antarctic Circle, considerable daylight in summer. Most of these factors vary both predictably and unpredictably in spatial and temporal planes. In response to this adverse environment, fellfield organisms have developed a variety of strategies to overcome physiological stress and to exploit the limited resources available during the short austral growing season. A high degree of synchronisation exists, so that investment in non-essential activity and adaptations is minimised. Here, we review the combined suites of co-adapted traits used by different fellfield taxa to achieve energy acquisition, growth and reproduction under adverse levels of two principal limiting factors: low temperatures and the scarcity of water. To this end, a detailed characterisation of the Antarctic fellfield microenvironment is followed by a synthesis of available data on the morphology, physiology, life history and behaviour of successful Antarctic flora and fauna. Tolerance of low temperatures by fellfield organisms is achieved by elevation of standard metabolism, production and accumulation of cryoprotectants, supercooling, melanic pigmentation, behavioural avoidance, compact growth forms and synchronised reproduction and extended life cycles. Low moisture conditions are overcome by dehydration resistance, anhydrobiosis, development of resting stages and by behavioural avoidance of desiccating conditions. Occupancy of the Antarctic fellfield habitat is considered to require the ability to respond rapidly to ephemeral resources and to tolerate severe environmental stresses. During summer, organisms rely on opportunism to maintain a positive energy balance. During winter, resistance adaptations are used to withstand the potentially lethal climate, especially in habitats not protected by snow cover. This deterministic framework has led to the selection of species that are genetically and physiologically pre-adapted for resource acquisition yet sufficiently robust to withstand cold and desiccation stresses. Non-adapted taxa fail to become established. Despite the environmental selection pressures, available evidence suggests that colonisation of the fellfield habitat has not required the evolution of any adaptations, only the refinement of those already possessed to an extent by some temperate forms. This has led to the convergence of survival strategies. It is hypothesised that, in the short term, the majority of Antarctic fellfield biota are able to absorb the predicted effects of a changing climate by their high levels of physiological tolerance and life-cycle flexibility.

Effect of lifespan and age on reproductive performance of the tardigrade Acutuncus antarcticus: minimal reproductive senescence

“…Asexual reproduction is considered an ancestral feature present in many of the major groups of invertebrates that are components of ecosystems in more extreme environments including Antarctica (Convey, 2010). Life cycle extension is widely used by many taxa to survive in low-energy and 'adversity selected' environments such as those of Antarctica in order to overcome insufficient time to complete a full reproductive cycle within one season (Convey, 1996;Block et al, 2009). While prolonged survival has been hypothesised to be sacrificed for higher reproduction at early ages in some unfavourable environments (Kirkwood & Rose, 1991), prolonged maintenance of reproductive ability also gives an advantage in extended life cycles in the natural environment of Antarctica.…”

Responses of invertebrates to temperature and water stress: A polar perspective

“…In few places are invertebrates more directly impacted by their environment than in the Arctic tundra 47 (Strathdee and Bale 1998) ice where it is inaccessible (Block et al 2009) while, in summer, streams, lakes and rock pools, which 60 form from melted ice and snow in spring, evaporate, resulting in drought . Again, 61 behavioural responses can help reduce desiccation stress (Hayward et al, 2000(Hayward et al, , 2001.…”

Contrasting strategies of resistance vs. tolerance to desiccation in two polar dipterans

“…Water availability is limited for much of the year in the Arctic and Antarctic (Strathdee & Bale 1998;Block et al 2009), and polar terrestrial invertebrates respond with one of two strategies, desiccation resistance or desiccation tolerance. Our data suggest that the Arctic dipteran Heleomyza borealis responds through desiccation resistance, while the Antarctic species Eretmoptera murphyi tolerates substantial desiccation (Figs.…”

“…In winter, water is locked up as ice and is inaccessible to invertebrates (Block et al 2009), while in summer, evaporation of meltwater can lead to drought (Kennedy 1993). In some areas, such as the McMurdo Dry Valleys, soil water content can be as little as 2% (Treonis & Wall 2005).…”

“…Low water availability is seen as being one of two principal stressors to terrestrial invertebrates in the polar regions, with the other being low temperature (Cannon & Block 1988;Convey 1996;Strathdee & Bale 1998;Block et al 2009). In winter, water is locked up as ice and is inaccessible to invertebrates (Block et al 2009), while in summer, evaporation of meltwater can lead to drought (Kennedy 1993).…”